gives partial protection . In addition, the extent of the velocity effect is dependent on the exposure time. The velocity below which a completely protective scale forms is defined as the critical velocity . The weight loss of stainless-steel specimens above this velocity increases linearly with time, and the corrosion rate is constant . Below the critical velocity the stainless steel corrodes initially at the same rate as at the high velocities, but the rate decreases as the protective coating forms and, generally, after about 100 hr very little, if any, corrosion occurs . In fact, some specimens have been exposed continuously at flow rates less than the critical velocity for periods of time as long as 20,000 hr and the amount of metal corroded was no greater than after 100 hr [29] . 5-4 .4 Effect of uranyl sulfate and sulfuric acid concentration . All uranyl-sulfate solutions are acid, and the more concentrated the solution, the lower the pH as measured at room temperature . The acidity is further increased by adding sulfuric acid to dilute uranyl sulfate solutions to prevent hydrolytic precipitation of copper and uranium at high temperature . Generally, the higher the concentration of uranyl sulfate or free acid in solution, the greater the extent of metal dissolution during film formation below the critical velocity, the lower the critical velocity, and the higher the film-free corrosion rate above the critical velocity . Table 5-7 shows how the above three regions change with uranyl-sulfate concentrations at 250 째C.
TABLE 5 - 7 THE CORROSION OF TYPE-347 STAINLESS STEEL IN URANYL SULFATE SOLUTIONS AT 250 째 C
Concentration of UO 2SO.,, m
Wt. loss at 10 fps, mg/cm2
Critical velocity, fps
Corrosion rate at 60 fps, mpy
0 .02 0 .11 0 .17 0 .43 0 .84 1 .3
1-2 2-3 12 20 37 50
> 50 25-30 20-25 10-20 10-20 10-20
-10 190 190 400 680 1400